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LTC3610 Datasheet(PDF) 11 Page - Linear Technology |
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LTC3610 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC3610 11 3610fd APPLICATIONS INFORMATION The basic LTC3610 application circuit is shown on the front page of this data sheet. External component selection is primarily determined by the maximum load current. The LTC3610 uses the on-resistance of the synchronous power MOSFET for determining the inductor current. The desired amount of ripple current and operating frequency also determines the inductor value. Finally, CIN is selected for its ability to handle the large RMS current into the converter and COUTischosenwithlowenoughESRtomeet the output voltage ripple and transient specification. VON and PGOOD The LTC3610 has an open-drain PGOOD output that indicates when the output voltage is within ±10% of the regulation point. The LTC3610 also has a VON pin that allows the on-time to be adjusted. Tying the VON pin high results in lower values for RON which is useful in high VOUT applications. The VON pin also provides a means to adjust the on-time to maintain constant frequency operation in applications where VOUT changes and to correct minor frequency shifts with changes in load current. VRNG Pin and ILIMIT Adjust The VRNG pin is used to adjust the maximum inductor valley current, which in turn determines the maximum average output current that the LTC3610 can deliver. The maximum output current is given by: IOUT(MAX) = IVALLEY(MAX) + 1/2 ΔIL, The IVALLEY(MAX) is shown in the figure “Maximum Valley Current Limit vs VRNG Voltage” in the Typical Performance Characteristics. An external resistor divider from INTVCC can be used to set the voltage on the VRNG pin from 0.5V to 1V, or it can be simply tied to ground force a default value equivalent to 0.7V. When setting current limit, ensure that the junc- tion temperature does not exceed the maximum rating of 125°C. Do not float the VRNG pin. Operating Frequency The choice of operating frequency is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses but requires larger inductance and/or capacitance in order to maintain low output ripple voltage. The operating frequency of LTC3610 applications is de- termined implicitly by the one-shot timer that controls the on-time tON of the top MOSFET switch. The on-time is set by the current into the ION pin and the voltage at the VON pin according to: t V I pF ON VON ION = () 10 Tying a resistor RON from VIN to the ION pin yields an on-time inversely proportional to VIN. The current out of the ION pin is: I V R ION IN ON = For a step-down converter, this results in approximately constant frequency operation as the input supply varies: f V VR pF H OUT VON ON Z = () [] 10 To hold frequency constant during output voltage changes, tie the VON pin to VOUT or to a resistive divider from VOUT when VOUT > 2.4V. The VON pin has internal clamps that limit its input to the one-shot timer. If the pin is tied below 0.7V, the input to the one-shot is clamped at 0.7V. Similarly, if the pin is tied above 2.4V, the input is clamped at 2.4V. In high VOUT applications, tying VON to INTVCC so that the comparator input is 2.4V results in a lower value for RON. Figures 1a and 1b show how RON relates to switching frequency for several common output voltages. |
Similar Part No. - LTC3610 |
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Similar Description - LTC3610 |
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